The present invention relates to current sources, and more particularly, to a constant current source using CMOS technology.
Typically, the use of NMOS components made it relatively easy to realize a constant current source. FIG. 1 shows an NMOS transistor 100 configured as a typical constant current source. The gate terminal 102 is connected to the source terminal 104 so that the gate to source voltage (Vgs) equals zero. Since the transistor 100 has a negative voltage threshold (Vt), constant drain current is available. This typical constant current source has been widely used to make an RC time constant that is independent of the power supply (Vcc).
FIG. 2 shows an RC circuit 200 that uses the typical constant current source circuit of FIG. 1. The circuit 200 includes NMOS transistor 202 that forms the constant current source and transistor 204 that receives a control input 206. The circuit 200 also includes a timing capacitor 208 couple to output 210. During operation, the constant current source 202 provides constant current to charge the timing capacitor 208 and thereby form a consistent output signal.
FIG. 3 shows graph 300 of the output of the RC circuit 200 of FIG. 2 plotted against time. As indicated at 302, when the input signal is at a high level (in H), the output signal approaches zero. As indicated at 304, when the input signal is at a low level (in L), the output signal rises at a constant rate as a result of the constant current source providing constant current. For example, dv/dt at the output is a constant, since a constant current is received by the capacitor 208. Thus, the constant current source allows many useful circuits to be constructed, such as the constant current RC circuit.
However, as technology has migrated to utilizing CMOS processes and components, the negative threshold NMOS transistor has become generally unavailable. Therefore, it is desirable to have a way to obtain a constant current source utilizing CMOS technology.
The present invention includes a system for providing a constant current source utilizing CMOS technology. The system includes a CMOS circuit that replaces typical NMOS circuits to produce a constant current.
In one embodiment of the invention, a CMOS circuit that operates as a constant current source is provided. The circuit comprises a bias circuit that includes a bias output terminal. The bias circuit produces a bias signal that is output at the bias output terminal. The circuit also includes a switch circuit having a switch input terminal coupled to the bias output terminal to receive the bias signal. The switch circuit also includes a switch output terminal and a switch control terminal that is coupled to receive an input signal, and wherein the bias signal is switched to the switch output terminal to form a switched bias signal in response to the input signal. The circuit also includes an output circuit having a first input terminal coupled to the switch output terminal to receive the switched bias signal, a second input terminal coupled to receive the input signal and an output terminal. The output circuit operates to produce an output signal that has constant current at the output terminal.
In one embodiment of the invention, a method for generating constant current from a CMOS circuit is provided. The method comprises the steps of generating a bias signal, switching the bias signal in response to an input signal, wherein a switched bias signal is produced, and receiving the input signal and the switched bias signal to produced an output signal that provides constant current.